A multi-carrier communication system, such as a Discrete Multiple-Tone (DMT) system in the various types of Digital Subscriber Line (e.g. ADSL and VDSL) systems, carries information from a transmitter to a receiver over a number of tones. Each tone may be a group of one or more frequencies defined by a center frequency and a set bandwidth. The tones are also commonly referred to as sub-carriers or sub-channels. Each tone acts as a separate communication channel to carry information between a local transmitter-receiver device and a remote transmitter-receiver device.
DMT communication systems use a modulation method in which the available bandwidth of a communication loop, such as twisted-pair copper media, is divided into these numerous sub-channels. A communication loop may also be known as a communication channel. However, to avoid confusion, the term channel is used herein in reference to tones and frequencies, rather than transmission medium. The term communication loop is understood to refer generally to a physical transmission medium, including copper, optical fiber, and so forth, as well as communication signal paths, including radio frequency (RF) and other physical or non-physical communication signal paths.
There are various sources of interference and noise in a multi-carrier communication system. Interference and noise may corrupt the data-bearing signal on each tone as the signal travels through the communication loop and is decoded at the receiver. The transmitted data-bearing signal may be decoded erroneously by the receiver because of this signal corruption.
In order to account for potential interference on the transmission line and to guarantee a reliable communication between the transmitter and receiver, each tone can merely carry a limited number of data bits per unit time. This number is related to a bit error rate (BER) for a given tone. The number of data bits or the amount of information that a tone carries may vary from tone to tone and depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases.
Many communication systems are designed to operate in a duplex mode. In this mode, a single transmission medium is used for both receive and transmission of data. For instance, in DSL systems, a single pair of twisted wires is used to carry information in both directions. In such cases, the modem has to separate the effects of its own transmit signal from the incoming signal received from the line. Typically, a near end signal and a far end signal are separated to create an upstream signal and a downstream signal. The near end signal refers to the signal send by the local transmitter. The far end signal refers to the signal sent by a remote transmitter. If the seperation is not done properly, the near-end transmit signal manifests as a transmission echo corrupting the received far-end signal. The corrupting echo should be minimized in order to achieve the best data rate.